1. Field of the Invention
The present invention relates to a control system for an aircraft engine and, more specifically, it relates to a control system for a turbo-charged diesel engine used for an aircraft equipped with a controllable pitch propeller.
2. Description of the Related Art
A controllable pitch propeller for an aircraft that can change its pitch during operation is known in the art. In an aircraft equipped with a controllable pitch propeller, the engine speed and the engine output power are usually controlled separately. Namely, the aircraft equipped with a controllable pitch propeller is normally provided with a propeller governor that automatically changes the propeller pitch so that the propeller speed becomes a predetermined set speed. Since the power consumption of the propeller changes in accordance with the propeller pitch and the propeller speed, the engine speed (i.e., the propeller speed) changes in accordance with the propeller pitch if the engine output power is kept at constant value.
Further, when a diesel engine is used, engine output is controlled separately from the engine speed by adjusting the amount of fuel injected into the engine. Therefore, in the conventional control system, the pilot of the aircraft is required to adjust a governor lever for changing the set speed of the propeller governor and an accelerator lever for changing the setting value for fuel injection amount (a set amount of fuel) simultaneously in order to obtain a desired engine (propeller) speed and engine output.
However, in the conventional control system, as the pilot is required to operate the governor lever and the accelerator lever at the same time, controlling the operation of the aircraft has become complicated.
To solve this problem, U.S. Pat. No. 5,810,560 proposes a control system for an aircraft engine in which both the set speed of propeller governor and the set amount of fuel injection is controlled by a single control lever. In U.S. Pat. No. 5,810,560, as a gasoline engine is used, the engine output is controlled by adjusting the degree of opening of a throttle valve disposed on the air intake passage of the engine, and the propeller governor and the throttle valve are connected to a single control lever by means of link and cam mechanism, and the set speed of the propeller governor and the degree of opening of the throttle valve change simultaneously in accordance with the stroke of the control lever. Therefore, when the set speed of the propeller governor becomes low, the degree of opening of the throttle valve is also set at a small value, whereby a low speed and low output power operation of the engine is automatically achieved. Similarly, when the set speed of the propeller governor becomes high, as the degree of opening of the throttle valve also becomes large, a high speed and high output power operation of the engine is automatically achieved.
According to the control system in U.S. Pat. No. 5,810,560, as the engine speed and the engine output power are controlled by a single lever, the complexity of controlling the aircraft is largely reduced.
However, when the control system of U.S. Pat. No. 5,810,560 is used for a turbo-charged diesel engine, some problems occur.
In a turbo-charged diesel engine, the intake air amount of the engine is not controlled by a throttle valve and determined by the engine speed and the boost pressure. Therefore, a combustion air-fuel ratio changes in accordance with the fuel injection amount, i.e., the engine load. Consequently, in some operating conditions, if the combustion air-fuel ratio becomes excessively low, exhaust smoke will be formed due to a shortage of combustion air.
In order to prevent the formation of exhaust smoke, a boost compensator is used in some turbo-charged diesel engines. The boost compensator is an apparatus that restricts the fuel injection amount to a value less than a maximum limit (i.e., so called xe2x80x9ca smoke limitxe2x80x9d) corresponding to an allowable lowest air-fuel ratio. The allowable lowest air-fuel ratio is a lowest air-fuel ratio on which the engine can operate without forming exhaust smoke. Since the amount of air charged into cylinders of the engine increases as the boost pressure increases, the maximum limit of the fuel injection amount is determined by the boost pressure.
When the engine speed and the fuel injection amount are simultaneously controlled by a single control lever, the fuel injection amount changes in accordance with the engine speed. In this case, when the engine speed is determined, the fuel injection amount is simultaneously determined. On the other hand, the boost pressure decreases as the altitude increases even if the engine speed is constant, due to a decrease in the atmospheric pressure. Therefore, in some cases, the fuel injection amount exceeds the smoke limit at a high altitude even if the engine speed is not changed.
If the boost compensator as explained above is used in the single control lever system, the boost compensator is activated to restrict the fuel injection amount at a high altitude due to a decrease in the boost pressure, even if the engine is operated in steady operating conditions. In a steady operation of the engine, once the boost compensator restricts the fuel injection amount, boost pressure does not increase any more since sufficient engine power for increasing the boost pressure is not available. Therefore, if the boost compensator is activated in a steady operation of the engine, the fuel injection amount would likely always be controlled to the maximum amount determined by the boost pressure at a high altitude. When the fuel injection amount is controlled by the boost compensator, the fuel injection amount changes in accordance with the boost pressure. This means that, if the boost pressure increases by a small amount, the fuel injection amount is increased by the boost compensator in accordance with the increase in the boost pressure. When the fuel supplied to the engine increases, the boost pressure also increases, due to an increase in the energy supplied to the turbocharger, and the increase in the boost pressure further increases the fuel injection amount.
Therefore, when the fuel injection amount is controlled by the boost compensator during a steady operation of the engine, even a small fluctuation in the boost pressure is amplified to a large change in the boost pressure and the fuel injection amount. In other words, the operation of the engine becomes unsteady.
Therefore, when the single control lever system is used, it is considered not preferable to use the boost compensator.
In view of the problems in the related art as set forth above, an object of the present invention is to provide a control system for a turbo-charged diesel aircraft engine using a single control lever for controlling an engine speed and a fuel injection amount of the engine and capable of preventing the formation of the exhaust gas smoke without causing instability of the operation of the engine.
The object as set forth above is achieved by a control system, according to the present invention, for a turbo-charged diesel aircraft engine comprising a controllable pitch propeller connected to and driven by the turbo-charged diesel aircraft engine, a propeller governor for controlling the rotational speed of the propeller to a set speed by adjusting the pitch of the propeller, speed setting means for changing the set speed of the propeller governor, accelerator means for changing a set amount of fuel supplied to the engine, fuel supply means for supplying fuel to the engine by an amount determined in accordance with-the set amount, control means for controlling the speed setting means and the accelerator means so that the set speed and the set amount of fuel change simultaneously in accordance with the stroke of a single control lever, and wherein the control means controls the speed setting means and the accelerator means in such a manner that the set speed and the set amount of fuel maintains a predetermined fixed relationship in which the set amount of fuel always becomes smaller than a smoke limit value at the maximum design altitude of the aircraft and determined by the set speed.
According to the present invention, as the fuel injection amount is always set at a value smaller than a smoke limit of the engine at the maximum design altitude of the aircraft. The smoke limit is a maximum amount of fuel that can be fed to the engine without producing exhaust smoke. The smoke limit changes in accordance with the boost pressure of the engine. Therefore, usually, the smoke limit value becomes larger as the set speed of the engine becomes higher. Further, the boost pressure decreases as the altitude of the aircraft increases even if the engine speed is kept constant. Therefore, the smoke limit value becomes smaller as the altitude increases. Consequently, the smoke limit at the maximum design altitude is the smallest value of the smoke limit at the respective set speeds.
In the present invention, the set amount of fuel is changed in accordance with the set speed of the engine, further, the set amounts of fuel at the respective set speeds are kept at a value smaller than the smoke limit value at the maximum design altitude. Since the smoke limit at the maximum design altitude is the smallest value of the smoke limits at the respective set speeds, the set amount of fuel in the present invention never exceeds the smoke limit at any altitude. Thus, according to the present invention, the formation of exhaust smoke is suppressed over the whole altitude range of the aircraft operation.
Further, the control system may be provided with a boost compensator and an altitude compensator. The boost compensator restricts the set amount of fuel determined by the operation of the single control lever to a value less than the smoke limit determined by the boost pressure. Therefore, if the boost compensator is provided, the formation of the exhaust smoke can be effectively suppressed even in a transient operation (such as an acceleration) of the engine. Further, the altitude compensator restricts the set amount of fuel determined by the operation of the single control lever to a value less than a maximum amount of fuel to avoid overrunning the turbocharger. This maximum amount of fuel (an altitude fuel limit) is determined in accordance with the altitude of the aircraft. Therefore, if the altitude compensator is provided, the set amount of fuel is always kept at a value smaller than the altitude fuel limit and overrunning of the turbocharger does not occur.